1. Field of the Invention
The present invention relates to a multi charged particle beam writing apparatus and a multi charged particle beam writing method. More specifically, for example, the present invention relates to correcting the space charge effect in multi beam writing.
2. Description of Related Art
The lithography technique that advances miniaturization of semiconductor devices is extremely important as a unique process whereby patterns are formed in semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. The electron beam (EB) writing technique, which intrinsically has excellent resolution, is used for writing or “drawing” a pattern on a wafer and the like with electron beams.
As an example employing the electron beam writing technique, a writing apparatus using multiple beams (multi beams) can be cited. Compared with the case of writing a pattern by using a single electron beam, since it is possible to emit multiple beams at a time in multi-beam writing, the throughput can be greatly increased. For example, in a writing apparatus employing a multi-beam system, multiple beams are formed by letting an electron beam emitted from an electron gun assembly pass through a mask with a plurality of holes, blanking control is performed for each of the beams, and each unblocked beam is reduced by an optical system and deflected by a deflector so as to irradiate a desired position on a target object or “sample” (refer to, e.g., Japanese Published Unexamined Patent Application (JP-A) No. 2006-261342).
In the multi-beam writing, when performing highly precise writing, the dose of each beam is individually controlled based on an irradiation time in order to give a specified dose onto each position on a target object. With respect to each shot, since the irradiation time of each beam is not constant, even if 10000 beams are emitted at a certain time in one shot, only 1 ns (nanosecond) later, for example, the number of ON beams becomes 500, and further 1 ns later, the number of ON beams becomes 50, for example. Thus, since the number of ON beams of multi beams changes every moment, the ON-beam pattern and the beam current in the multi beams change dynamically in one shot. Therefore, in a charged particle beam, such as an electron beam, repulsion occurs, on the beam path, among particles having electric charge and thereby the beam path changes. The effect of this causes a space charge effect that becomes large in proportion to the beam current (the number of particles). Consequently, there is a problem that a focus deviation, a position deviation of a writing pattern and the like are generated due to the space charge effect during irradiation of each beam in one shot.
However, it is difficult to perform correction of a focus deviation, a position deviation of a writing pattern and the like such that the correction follows an ON-beam pattern and a beam current that change dynamically during irradiation of each beam in one shot.